Malachite after Azurite (#37)

Dull opaque green rosettes to 1 ¼" aesthetically grouped together on limonite.
South America Seabra, Bahia, Brazil 4 ½" x 3 ¾" x 2 ¼"

 

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Malachite alter azurite? Not malachite and azurite, but malachite attar azurite. What does this mean? This is a mineralogical way of saying that azurite formed first and then was replaced by malachite. We call this pseudomorphism - literally "false form" - and pseudomorphs of one mineral after another are rather common. Of course, to know that is the case, one must recognize that the mineral that is present has the crystal shape of some other mineral. It has replaced the original mineral but retained the first mineral's form. The transformation of malachite to azurite occurs by a reaction that can be represented this way:

     2 azurite + 1 water → 3 malachite + 1 carbon dioxide

or

     2[Cu3(CO3)2(OH)2] + H2O → 3[Cu2(CO3)(OH)2] + CO2

This reaction takes place on a very small spatial scale a little at a time, with the result that the shape of the original azurite crystals is not disrupted. Pseudomorphism proceeds because either the chemical environment or the physical conditions (temperature, pressure) are more favorable to the replacement mineral than to the original mineral, assuming that the necessary energy is present in the system to cause the transformation.